While not as immediately destructive as traditional explosives, they could ultimately prove far more devastating in terms of casualties, as they have the potential to spread radioactive material over a wide area, possibly leading to cancer and radiation poisoning.

Professor Wilkinson told BBC Radio 4's Today programme that the original home office warning was "not inaccurate".

Text of revised statementClick here to read

"I suspect that really the reason for its withdrawal was that they did not want to highlight specific threats. "They wanted to point out that the threat was from a whole variety of tactics," he added.

Simon Hughes, Liberal Democrat home affairs spokesman, called on Home Secretary David Blunkett to clear up the "confusion".

Bali

The summary was published as Mr Blunkett and British intelligence heads met US homeland security chief Tom Ridge, to discuss anti-terror measures taken by both countries.

In a speech at King's College in London on Thursday evening, Mr Ridge said the threat from al-Qaeda and other groups was "unlike any other we have faced".

He said the recent bombing in Bali, the attack on the French oil tanker, the killing of Europeans in Pakistan and the death of a US Marine in Kuwait indicated al-Qaeda could both orchestrate attacks and inspire sympathisers.

And he warned of "tens of thousands" of casualties if transportation networks, power generating plants or health infrastructures were overwhelmed by an attack.